The ability of a photoprotective composition to provide protection against UV radiation is typically expressed as the sun protection factor (SPF) of the composition. Photoprotective compositions having SPF values of 80 or more generally contain 15% homosalate, which is an acceptable amount of homosalate for compositions in the U.S. market. However, compositions in the international market are allowed to include only 10% homosalate. To maintain an SPF of 80 or more for the products available in the international market, it is therefore desirable to limit the homosalate thereof to 10%.
Zinc oxide has been indicated as being a material that is inherently pyroelectric. Pyroelectricity is the ability of a certain material to generate an electrical potential when the material is heated or cooled. As a result of this change in temperature, positive and negative charges migrate to opposite ends of the zinc oxide lattice structure (the material becomes polarized). Thus, an electrical potential is established.
Zinc oxide is also a semiconductor with a direct band gap energy (Eg) of 3.37 eV at room temperature. Most zinc oxide has n-type character (as opposed to p-type character, which is more difficult to attain), which means that electron energy levels near the top of the band gap allow an electron to be excited into the conduction band with relative ease. Native defects such as oxygen vacancies or zinc interstitials are often assumed to be the origin of this n-type character. Intentional doping of the n-type zinc oxide, which may be effected by introducing aluminum, indium, or excess zinc into the zinc oxide structure, allows the zinc oxide to be formed into thin films in which the zinc oxide serves as a transparent conducting oxide, which can be used to form a transparent electrode.
When a photon hits zinc oxide, one of three things can happen: (1) the photon can pass straight through the zinc oxide, which happens for lower energy photons; (2) the photon can reflect off the surface; or (3) the photon can be absorbed by the zinc oxide. If the photon is absorbed by the zinc oxide, either heat or electron-hole pairs may be generated. Electron-hole pairs are generated if the photon energy is higher than the zinc oxide band gap value.
An incident photon may be absorbed by the zinc oxide if its energy is greater than the semiconductor band gap energy. As a result, an electron from the valence band of the zinc oxide (wherein the electron comes from the oxygen) is promoted into the conduction band (the metal ion orbital). If the energy of the incident photon is less than the band gap energy, it will not be absorbed as this would require that the electron be promoted to within the band gap. This energy state is forbidden. Once promoted into the conduction band, however, the electron relaxes to the bottom of the conduction band with the excess energy emitted as heat to the crystal lattice.
When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon “excites” the electron into the conduction band where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron (which results in the formation of a “hole”). The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the hole, thereby leaving another hole behind, and in this way a hole can “move” through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron/hole pairs.
The holes act as positive particles in the valence band. Both the electrons and the holes are free to migrate around the zinc oxide particle. Electrons and holes may recombine emitting photons of energy equal to the band gap energy. However, the lifetime of an electron/hole pair is quite long due to the specific nature of the electronic band structure. Thus, there is sufficient time for an electron and a hole to migrate to the surface and react with absorbed species.
A photon need only have a greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. In the solar frequency spectrum, much of the radiation reaching the surface of the earth is composed of photons with energies greater than the band gap of silicon (1.12 eV) and zinc oxide (3.37 eV). The higher energy photons will be absorbed by the difference in energy between these photons, and the band gap energy is converted into heat via lattice vibrations (called phonons).